1. Field of Invention
This invention relates to current generating cells, and more particularly to a current generating cell responsive to radiation, such as solar energy.
2. Description of the prior art.
A conventional silicon radiation responsive cell, having a characteristic response curve for energy, FIGS. 1 and 2, includes a body 10 having a first semiconductor P type conductivity region 12 and an overlying N+ conductivity semiconductor region 14 for forming a PN junction 16. For prior art structures, such as those used in space exploration, the cell 10 includes a starting substrate of silicon material approximately 10 mils thick. Thereafter an N+ diffusion into the starting semiconductor substrate results in the first P type region 12 and the overlying N+ region 14 approximately 0.5 microns thick. The back of the body 10 is coated with a metalized layer 17 and a plurality of upper surface metal contacts designated generally at 18 provide electrical contact to the PN junction 16. The upper surface contacts 18 are separated and formed of minimal dimensions in order to allow a maximum amount of incident radiation to be absorbed by the upper N+ region 14.
The prior art and preferred embodiments are illustrated for silicon material and solar radiation, however, it is to be understood that other radiation sources, such as infrared, and other materials can be used for forming the PN junction, for example, germanium, gallium phosphide, gallium arsenide, cadmium sulfide, etc. It is known that the bandgap for silicon is approximately 1.1 electron volts. Only radiation with energy greater than this energy will create electron hole pairs. As a result, only incident radiation of wavelength less than about 1.2 microns possesses sufficient energy to traverse this bandgap.
In FIG. 1 the incident radiation is illustrated by schematic rays 20 impinging upon the upper N+ region 14. The photons associated with the incident radiation 20 are effective to create electron hole pairs generally depicted at 22 and 24. The shorter wave lengths of incident radiation primarily form electron hole pairs in the upper region 14 and are not effective to create electron hole pairs deeper into the body 10. The longer wave lengths, that is in the region of 1.2 microns, are effective to not only create electron hole pairs in the upper region 14 but deeper into the P type region 12. Once the electron hole pairs are formed, the minority carriers in each of the regions 12 and 14 attempt to diffuse across the PN junction 16 for generating output current.
The longer the minority carrier lifetime the more assurance that the minority carriers are able to cross the PN junction 16 and contribute to the generation of output current. Ideally, all of the electron hole pairs generated in the upper N+ region 14 will diffuse across the PN junction 16. However due to surface imperfections some of the minority carriers associated with the upper N+ region 14 diffuse upwardly as opposed to across the PN junction and thus recombine at the upper surface 32 of region 14. Consequently, the efficiency of the cell is diminished. Solar cells of the prior art and also of the present invention can be formed by numerous processes such as ion implantation, diffusion, or epitaxial processes. Similarly, heterojunction devices formed, for example, with germanium and gallium arsenide, gallium arsenide and gallium aluminum arsenide, etc. also can be employed to form the PN junction.
In the case of diffused junctions, the high impurity concentration level at the surface of the device, by its very nature, tends to produce a field gradient which will assist the minority carriers in the upper N+ region in diffusing across the PN junction rather than diffusing in the opposite direction and recombining at the upper surface 32. However, this inherent advantage of device profiling tends to decrease the lifetime of the minority carriers in that region and thus offsets the advantages of producing a field gradient by impurity profiling of the device itself.
In addition to profiling it is well known in other semiconductor areas, such as, MOS, that an applied potential forms an electric field in order to vary the conductivity of the channel existing between source and drain. In the typical structures, the applied potential generates a charge which is of the same polarity as the channel material in order to change its conductivity type and thus provide a top upper surface conductive channel between the source and drain.
Literature and prior art also mentions the fact that the existence of an oxide layer often causes a problem in that positive charges inherent in some SiO.sub.2 layers tend to invert P type material thus causing undesirable shorting or leakage paths between other N type regions located on the substrate.
Accordingly, improving the efficiency of a solar radiation cell by profiling so to introduce a field gradient is limited in practice because the profiling approach shortens the minority carrier lifetime and offsets some of the advantages gained by the built-in field gradient. As previously discussed, an electric field is either intentionally or intrinsically created for converting a semiconductor region of one conductivity type to an opposive conductivity type material.